One important feature of third generation cellular systems is to provide services over a wide range of data rates. In IS-2000 and wideband CDMA (WCDMA), service bearers of various data rates are achieved by using a combination of multi-code, multi-carrier, and/or multi-spreading factor. For example, in WCDMA, the spreading factors of physical channels vary from 256 to 4, corresponding to 15K symbols per second to 0.96 M symbols per second gross data rate. If multi-code is used with spreading factor 4 and quadrature phase shift keying (QPSK) modulation, a gross data rate higher than 2 M bits per second (bps) can be obtained.
However, in such a scenario, the conventionally used RAKE receiver does not perform well in a dispersive channel. This is because the processing gain, provided through signal spreading, is not high enough to reject inter-symbol interference (ISI) due to multipath. The ISI can come from adjacent symbols on the same code, or from overlapping symbols on other codes. As a result, user throughput and coverage are limited by multipath delay spread. As high-speed data communications become more and more important for future applications, it is critical to address this ISI problem when multi-code is used along with a low spreading factor, to guarantee adequate receiver performance even in dispersive channels.
In U.S. Pat. No. 6,975,672, receivers for detecting a direct-sequence spread spectrum (DS-SS) signal of a very low spreading factor were proposed. First, a maximum likelihood sequence estimator (MLSE) in additive white Gaussian noise (AWGN), which utilizes the Ungerboeck metric, was proposed. That application's proposed receiver structure is similar to one used for a narrow-band signal, except that the receiver parameter was to be recalculated for every symbol, accounting for the symbol-dependent scrambling code typically used in CDMA systems. The complexity of an MLSE receiver grows as the delay spread increases. To make the receiver complexity manageable, sub-optimal receivers, such as the decision feedback sequence estimator (DFSE), decision feedback equalizer (DFE), and reduced-state sequence estimator (RSSE), were introduced. The aforementioned MLSE, DFSE, DFE, and RSSE receiver structures were also extended to address the issue of noise temporal correlation when colored noise is encountered. It was shown that a generalized RAKE (G-RAKE) structure was used when colored noise was encountered.
Prior art receivers generally address single code reception. To maximize data throughput, multi-code may be used along with low spreading factors. Thus, a receiver capable of dealing with multi-code interference is highly desirable. Further, prior art receivers generally do not address the issue of soft value generation. In wireless data communications systems, typically, a forward error correction (FEC) code is used to improve the accuracy of receivers. To maximize the effectiveness of the FEC code, soft values corresponding to the log-likelihood ratio of the encoded bits are needed for the FEC decoder.